bpf: adding support for mapinmap in libbpf

in this patch series i'm adding a helper for libbpf which would allow it to load map-in-map(BPF_MAP_TYPE_ARRAY_OF_MAPS and BPF_MAP_TYPE_HASH_OF_MAPS).
first patch contains new helper + explains proposed workflow second patch contains tests which also could be used as example of usage

v2->v3:
- fixing typo in patch description
- initializing inner_map_fd to -1 by default

v1->v2:
- addressing nits
- removing const identifier from fd in new helper
- starting to check return val for bpf_map_update_elem

Nikita V. Shirokov (2):
bpf: adding support for map in map in libbpf
bpf: adding tests for mapinmap helpber in libbpf

 tools/lib/bpf/libbpf.c                      | 11 +++-
 tools/lib/bpf/libbpf.h                      |  2 +
 tools/testing/selftests/bpf/Makefile        |  3 +-
 tools/testing/selftests/bpf/test_mapinmap.c | 49 +++++++++++++++++
 tools/testing/selftests/bpf/test_maps.c     | 82 +++++++++++++++++++++++++++++
 5 files changed, 145 insertions(+), 2 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/test_mapinmap.c

2.15.1

idea is pretty simple. for specified map (pointed by struct bpf_map) we would provide descriptor of already loaded map, which is going to be used as a prototype for inner map. proposed workflow:
1) open bpf's object (bpf_object__open)
2) create bpf's map which is going to be used as a prototype
3) find (by name) map-in-map which you want to load and update w/ descriptor of inner map w/ a new helper from this patch
4) load bpf program w/ bpf_object__load

inner_map_fd is ignored by any other maps aside from (hash|array) of maps

---
 tools/lib/bpf/libbpf.c | 11 ++++++++++-
 tools/lib/bpf/libbpf.h |  2 ++
 2 files changed, 12 insertions(+), 1 deletion(-)

index a01eb9584e52..7e130e0c8fc9 100644
@@ -163,6 +163,7 @@ struct bpf_map {
 	char *name;
 	size_t offset;
 	int map_ifindex;
+	int inner_map_fd;
 	struct bpf_map_def def;
 	__u32 btf_key_type_id;
 	__u32 btf_value_type_id;

@@ -653,8 +654,10 @@ bpf_object__init_maps(struct bpf_object *obj, int flags)
 	 * fd (fd=0 is stdin) when failure (zclose won't close
 	 * negative fd)).
 	 */
-	for (i = 0; i < nr_maps; i++)
+	for (i = 0; i < nr_maps; i++) {
 		obj->maps[i].fd = -1;
+		obj->maps[i].inner_map_fd = -1;
+	}
 
 	/*
 	 * Fill obj->maps using data in "maps" section.

@@ -1146,6 +1149,7 @@ bpf_object__create_maps(struct bpf_object *obj)
 		create_attr.btf_fd = 0;
 		create_attr.btf_key_type_id = 0;
 		create_attr.btf_value_type_id = 0;
+		create_attr.inner_map_fd = map->inner_map_fd;
 
 		if (obj->btf && !bpf_map_find_btf_info(map, obj->btf)) {
 			create_attr.btf_fd = btf__fd(obj->btf);

@@ -2562,6 +2566,11 @@ void bpf_map__set_ifindex(struct bpf_map *map, __u32 ifindex)
 	map->map_ifindex = ifindex;
 }
 
+void bpf_map__add_inner_map_fd(struct bpf_map *map, int fd)
+{
+	map->inner_map_fd = fd;
+}
+
 static struct bpf_map *
 __bpf_map__iter(struct bpf_map *m, struct bpf_object *obj, int i)
 {

index b1686a787102..0a0b7e0ed554 100644
@@ -293,6 +293,8 @@ LIBBPF_API void bpf_map__set_ifindex(struct bpf_map *map, __u32 ifindex);
 LIBBPF_API int bpf_map__pin(struct bpf_map *map, const char *path);
 LIBBPF_API int bpf_map__unpin(struct bpf_map *map, const char *path);
 
+LIBBPF_API void bpf_map__add_inner_map_fd(struct bpf_map *map, int fd);
+
 LIBBPF_API long libbpf_get_error(const void *ptr);
 
 struct bpf_prog_load_attr {

2.15.1

adding test/example of bpf_map__add_inner_map_fd usage

---
 tools/testing/selftests/bpf/Makefile        |  3 +-
 tools/testing/selftests/bpf/test_mapinmap.c | 49 +++++++++++++++++
 tools/testing/selftests/bpf/test_maps.c     | 82 +++++++++++++++++++++++++++++
 3 files changed, 133 insertions(+), 1 deletion(-)
 create mode 100644 tools/testing/selftests/bpf/test_mapinmap.c

index 57b4712a6276..a3ea69dc9bdf 100644
@@ -38,7 +38,8 @@ TEST_GEN_FILES = test_pkt_access.o test_xdp.o test_l4lb.o test_tcp_estats.o test
 	test_lwt_seg6local.o sendmsg4_prog.o sendmsg6_prog.o test_lirc_mode2_kern.o \
 	get_cgroup_id_kern.o socket_cookie_prog.o test_select_reuseport_kern.o \
 	test_skb_cgroup_id_kern.o bpf_flow.o netcnt_prog.o \
-	test_sk_lookup_kern.o test_xdp_vlan.o test_queue_map.o test_stack_map.o
+	test_sk_lookup_kern.o test_xdp_vlan.o test_queue_map.o test_stack_map.o \
+	test_mapinmap.o
 
 # Order correspond to 'make run_tests' order
 TEST_PROGS := test_kmod.sh \

2.15.1